I don't understand your use of the term "fair" in this context.
Supported ended for Civ3 "vanilla" on the Mac well before it did on the PC. For me, fair would be to get as close to equal footing as the Windows release as possible. And then Civ3 has proven to have serious legs on the Mac --
the price remains at $45 new, $33 used vs.
$20 new and
$9.25 used or
$5 on Steam for PC (used vanilla C3 is $29 vs. $0.99!!). The game has had a much longer shelf life on Mac than PC, but ironically the initial Mac version wasn't supported nearly as long as the PC's. Early Mac buyers have found themselves on the outside in terms of price twice, once for the typical Mac tax, which I'm not quite specifically lamenting here, and again for the Macsoft/Aspyr Civ 3 hot potato and early obsolescence. Or three times -- Civ3 remains one of the more popular games on the Mac, and early adopters [only] are unable to keeping playing on their screamin' new machines. Whoops. Downgrade time. :^) Wonder if SheepSaver and OS 9 would work...
So the situation is not exactly, if you get my meaning, "just as Firaxis did for Windows versions of the expansions". The price for Civ 3 on Mac isn't just horribly inflated, but also, perhaps "artificially inflated" isn't the right word... It's also been inflated in a way that can only happen when you're talking porting.
Edit: In brief, I don't think we could argue that it would have been better for whoever had the initial Civ to continue supporting it via add-on expansions and a common executable of some sort than what ended up happening, right? If what we have now is fair, having continuous support for all Civ 3 owner/players would have been more than fair, and better. I believe that because the game had to be ported, the possibility for MacSoft Civ 3 owners to hit a relatively early dead end was realized. /shrug You can also reasonably blame Apple, of course, as they do break bkwds compat in the OS much more quickly than Microsoft does. And you can argue that Aspyr's lack of support for OS 9 jips low end mac users and that Macsoft did a better job. ;^)
The question, I guess, and I'll stop now, is that Civ 3 was rewritten on the Mac afaict, and that this rewriting didn't happen on Windows... Did this re-port affect the Mac gamer negatively? My gut says yes, and the easiest symptom I can find is early support termination which may have influenced (??) the inability to play on 10.5. Actually, the easiest symptom is that Mac users couldn't buy expansions for Civ 3, but honestly I'm not sure Windows users could. They could for a while, right? /shrug
What the heck breaks MacSoft's Civ3 on 10.5? The world may never know. ;^) /edit
Don't worry. I'm not an Aspyr hater. Nor am I a MacSoft hater; I enjoy playing Civ3 on old hardware every so often, and I'm excited Aspyr seems to continue to have a winner with Civ3 to some degree. And as Glenda said once, the only company with the power to bring Mac gaming up to Windows is Microsoft, and nobody's holding their breath on that one.

I simply find the weird wrangling that is Mac porting particularly interesting (like Aspyr holding how many copies of Quake III?), and Civ 3 is one of the most interesting tales (imo) of the bunch. I'm just curious.
And fwiw,
here's the link I saw where Brad mentions PTW and Macsoft together. I'm betting you're right they hadn't started coding, but it looks like MacSoft considered making it. I wonder if Firaxis (what were they before Firaxis?) had any say in whether MacSoft had to keep working on supporting Civ3, and if they got to stop once Aspyr picked up C3C.
Thanks again for the replies and catching me up on some history!
